Vaihinger is throughout careful to distinguish fiction alike from hypothesis and dogma. He regards the distinction as, methodologically, highly important, though not always easy to make. The “dogma” is put forward as an absolute and unquestionable truth; the “hypothesis” is a possible or probable truth, such as Darwin’s doctrine of descent; the “fiction” is impossible, but it enables us to reach what for us is relatively truth, and, above all, while hypothesis simply contributes to knowledge, fiction thus used becomes a guide to practical action and indispensable to what we feel to be progress. Thus the mighty and civilising structure of Roman law was built up by the aid of what the Romans themselves recognised as fictions, while in the different and more flexible system of English laws a constant inspiration to action has been furnished by the supposed privileges gained by Magna Carta, though we now recognise them as fictitious. Many of our ideas tend to go through the three stages of Dogma, Hypothesis, and Fiction, sometimes in that order and sometimes in the reverse order. Hypothesis especially presents a state of labile stability which is unpleasant to the mind, so it tends to become either dogma or fiction. The ideas of Christianity, beginning as dogmas, have passed through all three stages in the minds of thinkers during recent centuries: the myths of Plato, beginning as fiction, not only passed through the three stages, but then passed back again, being now again regarded as fiction. The scientifically valuable fiction is a child of modern times, but we have already emerged from the period when the use of fiction was confined to the exact sciences. Thus we find fiction fruitfully flourishing in the biological and social sciences and even in the highest spheres of human spiritual activity. The Linnæan and similar classificatory systems are fictions, even though put forward as hypotheses, having their value simply as pictures, as forms of representation, but leading to contradictions and liable to be replaced by other systems which present more helpful pictures. There are still people who disdain Adam Smith’s “economic man,” as though proceeding from a purely selfish view of life, although Buckle, forestalling Vaihinger, long ago explained that Smith was deliberately making use of a “valid artifice,” separating facts that he knew to be in nature inseparable—he based his moral theory on a totally different kind of man—because so he could reach results approximately true to the observed phenomena. Bentham also adopted a fiction for his own system, though believing it to be an hypothesis, and Mill criticised it as being “geometrical”; the criticism is correct, comments Vaihinger, but the method was not thereby invalidated, for in complicated fields no other method can be fruitfully used.